Objectives: To quantify the frequency and cost of switching prescribed generic brand drugs for others at dispensation in pharmacies. Methods: We performed a descriptive cross-sectional study to compare prescription and dispensation of generic drugs in a primary care quota by using computerized data from medical consulting rooms and the Pharmaceutical Service Management Program in 2005. Results: Of 16,181 prescriptions, 3,759 generic prescriptions were included in the study, with a mean price of 7.61 euros as opposed to 8.01 euros for those dispensed in the retail pharmacy. In 62.9% of drugs, the brand prescribed by the doctor was not dispensed. In 65.1% of cases in which the brand was switched, the drug was changed for another generic drug with a higher price. Conclusions: Changing the prescribed generic drug was common in the quota studied with a tendency to dispense another drug that was more expensive, thereby increasing costs.